It is known that the cover of an airbag system in a dashboard or trim panel of a motor vehicle can be designed as a separate closure member with a hinged connection to the dashboard or trim panel. Such a separate member can correspond in appearance to a petite glove compartment door that is opened from the inside during a crash event by actuation of a related airbag deployment system. The separate member can be manufactured as a discrete subcomponent which is assembled to a host dashboard or trim panel as decorative trim. Such an assembly requires precise, labor intensive alignment to ensure an unobtrusive fit and the retention of the cover with the dashboard or trim panel in the event of an airbag deployment.
There have been attempts to solve this problem by incorporating the airbag cover as an integral part of the dashboard or trim panel wherein an opening line (e.g., tear line) for the airbag is formed in the dashboard or trim panel, eliminating the complexity and cost of a separate closure member. The opening line can be formed by punching or stamping the backing of the dashboard or trim panel out of sheet metal or by reducing the thickness of certain zones in the case of injection molded backings. If the thickness is reduced on the reverse side of the dash board or trim panel, the result is an “invisible” airbag cover.
It is conventional for many applications to produce a line-shaped weakening of material in order to create a predetermined breaking line which can be broken, in case of need, by the application of force in order to separate the adjoining parts of material from one another or to form an opening. It is always advantageous when such predetermined breaking lines have a constant resistance to breaking along their length so that a severing can be effected with constant force. For various applications it is required for technical reasons concerning safety that the braking resistance be predictable in a constant and reproducible manner. One such application, for example, is an airbag cover. For cosmetic reasons, it is also sometimes required that the predetermined breaking line be invisible to the naked eye of the vehicle passengers.
If the advantages of laser machining are to be used for producing such line-shaped weakened portions, it can become difficult to meet the demands for a reproducible, constant breaking resistance. A reproducible, constant breaking resistance essentially requires that the residual wall thickness in the area of the line-shaped weakening be producible in a constant and reproducible manner. This can be achieved by a uniform removal depth, provided the material has a constant thickness.
Methods of material removal with laser radiation are known. According to one approach, the effective laser radiation intensity is regulated by switching the laser on and off depending upon the detected heat radiation, which appears unsuitable for higher machining speeds. In order to improve accuracy, the removal depth is measured and taken into account for correcting limiting values. The measurement of the removal depth can be effected with an optical sensor working on the principle of triangulation. However, the application of this measurement principle is limited to cuts of vertical configuration, cut gap widths greater than the cross section of the measurement beam, and a linear cutting path. Moreover, like other measurement principles which detect removal depth, this measurement principle is not suited to indicate the remaining residual wall thickness if the material thickness is not constant.
Automotive interior trim covering materials such as vinyl and leather are relatively tough and difficult to sever, and also require a predetermined severing pattern for proper door panel opening such that known pre-weakening grooves have been formed in the trim cover in a predetermined pattern to ensure proper opening.
It has been previously proposed to provide an “invisible seam” installation in which the deployment door pattern is totally invisible to a person seated in the vehicle passenger compartment, and even faint outlines or “witness” lines are preferably avoided.
Scoring of the covering layer from the inside, if not done accurately, can over time become at least faintly visible from the exterior of the trim piece.
Fabrication of automotive interior trim pieces with pre-weakening grooving, particularly for invisible seam applications is thus a difficult manufacturing challenge.
First, the groove depth must be carefully controlled in order to achieve reliable rupture of the outer cover at exactly the right time during an air bag deployment event. If the groove is too shallow, the thickness of the remaining material may be too great, presenting excessive resistance to severing, and delaying air bag deployment. Conversely, if too little material remains, cracking may result over time, or at least allow the appearance of externally visible “witness” lines.
The pre-weakening effect may also be less effective if the grooves are molded-in during the process inasmuch as cutting into materials such as vinyl has a better pre-weakening effect compared to molding-in the groove during initial manufacture of the panel.
The high pressures employed during injection molding can cause a “crazing” effect at the thinned bridging material extending over the gap defined by the groove. This crazed zone is rendered more visible as the part is removed from the mold, particularly if the part is not completely cooled when it is being removed. The net effect is that the molded groove becomes visible on the exterior side.
It is difficult to accurately and reliably control the depth of mechanical cutting of component materials such as sheet vinyl and leather, since the material is variable compressed by the pressure of a cutting instrument. One known partial cutting procedure is intended to enable accurate control over the depth of the cut into a sheet of pliant plastic material such as a vinyl skin. However, a purely mechanical cutting operation still has other inherent accuracy limitations and is slow to execute. Also, some cover materials have irregular inside surfaces. For example, dry powder slush processes can create such irregularities. If the groove depth is constant, this results in an irregular thickness of the remaining material, leading to erratic performance as the resistance to opening pressure can vary significantly.
The groove width is also important, in that if a too narrow groove is cut into many plastics, a “self-healing” may occur, particularly at elevated temperatures in which the groove sides will re-adhere to each other, causing the pre-weakening effect to be erratic or neutralized. The required groove width also varies with the notch sensitivity of the material being pre-weakened.
A further difficulty is encountered in assembling the pre-weakened component to the interior trim structure so that the lines of pre-weakening are properly registered with the other components. For example, the vinyl skin in a skin and foam instrument panel must be accurately positioned on the instrument panel substrate and the deployment door substrate panels so that the pre-weakening lines are stressed as the door edges hinge out under pressure from the air bag.
This alignment requirement creates manufacturing difficulties and increased costs particularly since a variety of forms of instrument panel structures are employed, such as skin and foam, vinyl clad, hard plastic with a finished surface, and since a variety of forming techniques are employed, such as vacuum formed calendared plastic sheet, dry powder slush molded, injection molded, and the like. A leather covering may be substituted for the vinyl skin layer.
The criticality of performance of deployment door panels in air bag systems has necessitated extensive ongoing testing of pre-weakening grooves (i.e., tear lines) formed in vehicle trim panels to ensure maintenance of process control and unit-to-unit repeatability. However, known methods of measuring the depth profile of pre-weakening grooves required manual testing by punching out test samples of the trim panel skin selected for testing and manually optically measuring the notch depth in a two dimensional cross-sectional perspective using a standard microscope. This testing regimen is inherently destructive, wherein the trim panel selected for testing must be discarded as scrap.
Accordingly, it is an object of the present disclosure to provide a test apparatus and method enabling non-destructive testing of pre-weakening grooves (i.e., tear lines) formed in vehicle trim panels providing satisfactory groove dimensional data and the ability to return the tested trim panel to the production line.